Prognostic Meteorological Discussion
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000
FXUS02 KWBC 211606
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1205 PM EDT MON AUG 21 2017

VALID 12Z THU AUG 24 2017 - 12Z MON AUG 28 2017


...GUIDANCE EVALUATION/PREFERENCES...

MODELS AND ENSEMBLES SHOW GENERALLY GOOD AGREEMENT WITH RESPECT TO
THE LARGE SCALE PATTERN DURING THE MEDIUM RANGE PERIOD. THERE
REMAIN SOME FORECAST/FLOW DETAILS TO WORK OUT...ESPECIALLY TOWARDS
THE END OF THE PERIOD...BUT OVERALL MODELS DO SHOW BETTER THAN
AVERAGE AGREEMENT ON MOST SYSTEMS.

THE UPPER LEVEL TROUGH INITIALLY OVER THE GREAT LAKES AND THE
NORTHEAST IS SLOW TO MOVE OFFSHORE LATER THIS WEEK AND THE
WEEKEND. THERE IS GENERALLY GOOD AGREEMENT AMONGST EVEN THE
OPERATIONAL MODELS EXCEPT THE 00Z CMC WHICH APPEARS SLIGHTLY
QUICKER IN MOVING THE TROUGH OFFSHORE. OUT WEST...MODELS CONTINUE
TO SHOW GOOD AGREEMENT WITH A SHORTWAVE ENTERING THE PACIFIC
NORTHWEST/VANCOUVER DAY 3/THURSDAY AND CLOSING OFF AN UPPER LOW AS
IT PROGRESSES EASTWARD THROUGH WESTERN CANADA REACHING THE WESTERN
HUDSON BAY SHORE BY DAY 6/SUNDAY. AS THE UPPER LOW LIFTS MORE
NORTHWARD ON DAY 6 AND 7...THE SOUTHERN EDGE OF THE LARGER SCALE
TROUGH SPLITS AND MOVES EAST-SOUTHEASTWARD INTO THE UPPER
MIDWEST/GREAT LAKES BY DAY 7. IN BETWEEN THESE TWO SYSTEMS...A
STRONG RIDGE BUILDS AND THERE IS SOME DISAGREEMENT AS TO THE
STRENGTH OF THIS RIDGE AXIS WHICH MAY HAVE SOME INFLUENCE ON THE
PROGRESSION OF THE CLOSED LOW TO THE WEST BY THE END OF THE PERIOD.

MODELS AND ENSEMBLES ALSO CONTINUE TO SHOW GENERALLY GOOD
AGREEMENT THAT THE REMNANTS OF TROPICAL STORM HARVEY WILL CROSS
THE BAY OF CAMPECHE AND MOVE GENERALLY WEST/NORTHWEST INTO MEXICO
OR MAYBE EVEN FAR SOUTHERN TEXAS BY DAY 5. THE 06Z GFS TENDS TO
WANT TO LIFT THE REMNANTS INTO TEXAS WHILE THE 00Z ECMWF MAINTAINS
AND MORE WESTWARD TRACK INTO EASTERN AND NORTHERN MEXICO. WHILE
THE FORECAST AT THIS POINT LIES SOMEWHERE IN THE MIDDLE...THE
EXACT TRACK OF THE REMNANTS AND INTERACTION OF THIS MOISTURE WILL
A LINGERING BOUNDARY ALONG THE GULF COAST BY THE END OF THE PERIOD
WOULD HAVE IMPACTS ON SENSIBLE WEATHER ACROSS THIS REGION.

FOR ALL SYSTEMS...THE WPC FORECAST USES A BLEND OF THE OPERATIONAL
06Z GFS/00Z ECMWF WITH THE 00Z GEFS/ECMWF ENSEMBLE MEANS WITH MORE
OPERATIONAL INFLUENCE ON DAYS 3 AND 4...SLOWLY PROGRESSING TO A
MOSTLY ENSEMBLE MEAN BLEND BY DAY 7.


...SENSIBLE WEATHER HIGHLIGHTS...

EXPECT SHOWERS AND THUNDERSTORMS TO FOCUS ACROSS THE SOUTHEAST AND
THE GULF COAST ALONG A LINGERING STATIONARY BOUNDARY. FLORIDA
SHOULD ALSO SEE AN INCREASE IN RAINFALL AS A TROPICAL WAVE ALONG
THE COAST INTERACTS WITH THIS FRONTAL BOUNDARY. TO THE
NORTH...SHOWERS AND THUNDERSTORMS WILL DEVELOP ACROSS THE CENTRAL
U.S. AND THE NORTHERN PLAINS/UPPER MIDWEST AHEAD OF UPPER
TROUGHING IN WESTERN MEXICO AND EVENTUALLY DROPPING INTO THE
NORTHERN TIER. SHOWERS AND THUNDERSTORMS SHOULD ALSO DEVELOP
ACROSS THE SOUTHERN ROCKIES LATE IN THE WEEK WITH RETURN FLOW
REACHING THE SOUTHERN PLAINS BY THE WEEKEND.

ONE PLACE OF UNCERTAINTY IS WITH THE REMNANTS OF TROPICAL STORM
HARVEY AS IT MOVES INTO EAST CENTRAL MEXICO BY FRI/SAT. MOISTURE
ON THE NORTHERN FRINGE OF THE SYSTEM MAY BRING THE POTENTIAL FOR
HEAVY RAINFALL ACROSS SOUTH TEXAS AND INTO THE RIO GRANDE VALLEY.
THE MAGNITUDE AND HOW THIS PLAYS OUT THOUGH STILL STRONGLY DEPENDS
ON EXACTLY WHERE THE REMNANTS TRACK AND HOW THE MOISTURE INTERACTS
WITH THE LINGERING FRONT ALONG THE GULF COAST.

REGARDING TEMPERATURES ACROSS THE COUNTRY...WITH THE UPPER TROUGH
ACROSS THE GREAT LAKES/OHIO VALLEY/NORTHEAST...TEMPERATURES WILL
BE SEVERAL DEGREES COOLER THAN NORMAL LATER THIS WEEK AND THROUGH
THE WEEKEND. STRONG RIDGING OUT WEST WILL ALSO LEAD TO WARMER THAN
AVERAGE TEMPERATURES ACROSS MUCH OF THE WESTERN U.S. AND THE GREAT
BASIN/SOUTHWEST.


SANTORELLI



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